A thermochromatic glass has variable light transmittance depending upon ambient temperature, and decreases sunlight transmittance at high temperature to prevent external energy from being introduced into an interior, while increasing the sunlight transmittance at low temperature to promote introduction of external energy into the interior.
Thus, cooling load of a building is reduced at high temperature conditions, and heating load is reduced by a heating effect of natural lighting at low temperature conditions, thereby enabling efficient conservation of energy.
Such a thermochromatic glass is generally produced by coating a surface of glass with a thin film of vanadium dioxide (VO2).
Vanadium dioxide has high light transmittance due to characteristics of a semiconductor having a monoclinic structure at low temperature, and is converted into an orthorhombic structure having metallic characteristics and decreases in light transmittance, as temperature increases.
However, vanadium is a transition metal and thus vanadium dioxide has various polymorphs such as V2O3, V2O5, V2O7, etc. Thus, it is important to achieve clear formation of a crystal phase of vanadium dioxide having thermochromatic characteristics at low temperature.
To this end, as disclosed in US Patent Publication No. 4,400,412 and International Publication No. WO2008/009967, chemical vapor deposition (CVD) using a gaseous metal precursor is typically used to smoothly form a thin film of vanadium dioxide among a plurality of polymorphs. In addition, it is necessary for this process to heat a glass substrate up to a temperature of 400° C. to 700° C. while depositing a thin film.
Sputtering is generally used in order to achieve stable production of a large-area coating glass for buildings.
However, since the coating glass for buildings using a large-area glass substrate employs continuous deposition using an in-line sputter, it is difficult to heat the glass substrate during deposition and sputtering is generally performed at low temperature. This is because heating of a large-area glass substrate having several tens of square meters can cause not only a technical problem in that the glass substrate can be broken by thermal shock, but also subordinate problems in that energy is excessively consumed and a large amount of carbon dioxide is emitted.
Such process difficulty limits commercialization of the thermochromatic glass.
Therefore, there is a need for a technique capable of clearly forming a crystal phase of vanadium dioxide while stably performing low-temperature metal vapor deposition without any problem.